GM Rochester Quadrajet - Q-Jet City

The Carb Shop Shows Us How To Redo The Q

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GM Rochester Quadrajet - Q-Jet City

The first step in any carb rebuild is taking it apart. Remove the nine screws securing the air horn, and then use a small punch to push the accelerator pump pin in towards the air horn so you can remove the accelerator pump arm and rod. After you lift off the air horn, the accelerator pump can be pulled out. Save the spring from beneath it.

Removing the power piston comes next. The primary metering rods hang from the power piston, and you should be careful not to bend them when removing the piston.

A spring sits under the power piston and can be removed at this stage. If the spring is broken, replace it with the appropriate new spring (available from the Carb Shop).

Using needle-nose pliers, lift the bowl baffle out and put it aside to be reinstalled later.

Lift the float hinge up next, using needle-nose pliers. If the stock float is brass or plastic, it should be replaced with a nitrophyl float that will give your carb quicker float response time, and a float that won't sink.

Use a wide, flat-blade screwdriver to loosen the brass seat. Lift it out with pliers.

This is the adjustable part-throttle screw, which determines the lowest position of the metering rods at cruise speed. Remove it next and inspect it for wear. If it's in good shape, it does not require replacement.

Use a wide, flat-blade screwdriver to loosen the accelerator pump check-ball screw as well. Remove it with your trusty needle-nose pliers and then turn the carb upside down so the steel ball falls out. Don't lose it!

Disassemble the outside of the carb next. Remove the fuel fitting and filter. Don't throw away the spring-you'll need it later.

The next step is to remove the choke assembly. After removing the center screw from the choke housing, use the choke rod to hook the lever and lift it out.

Carefully note the orientation of the fast-idle lever and adjuster before removing it. It must be reinstalled in the same position to work properly.

Two or three screws secure the baseplate and need to be removed to take it off. Then slide the steel baffle out of the secondary venturis from the bottom.

The Carb Shop uses a heated ultrasonic cleaning tank to soak the Q-Jet clean for 15-20 minutes. You'll want to soak yours in a big can of carb cleaner overnight. Heavy varnish and stubborn crud can be brushed off with a stiff, chemical-resistant plastic brush.

Reassembly is done in the reverse order of disassembly. If you replaced the float, bend its hanger so that the top of the float sits approximately 1/4-inch below the top of the main body with the air horn removed. This will give you a rough float level adjustment to start out with. Further tweaking of the float hanger may be needed to get the fuel level correct.

After replacing the needle and seat, screw the adjustable part-throttle screw all the way down and then back it off two turns. Install the bowl baffle next, and then the power piston spring, piston, and metering rods can go in after it. Reinstall the accelerator pump check ball and screw, then carefully slip the new air horn gasket under the power piston bracket. You're now ready to move to the top.

Before bolting the top back on, reassemble the choke housing and lever using the choke rod to hold the lever in place. Don't forget to tighten all the air horn screws, including the two that are in the primary venturi.

Use side-cutters to grip and push the accelerator pump arm's pin back in place.

The Carb Shop seals the factory well plugs with super-tough epoxy. This is often a troublesome area of hidden internal leaks, and this cures the problem.

Most old carbs have throttle shafts that are looser than your wallet on date night; the Carb Shop machines the baseplate and installs oil-lite bronze bushings on the primary throttle shaft to tighten it up.

Here's something the Carb Shop does to make tuning your carb easier: They install a removable plug above the adjustable part-throttle screw to provide easier access for later adjustments.

Every one of the Carb Shop' s rebuilt Q-Jets gets bolted onto the Vrbancic Brother's Racing dyno for a full-throttle flogging before it gets shipped to your door. This big-block street Q-jet had no trouble cranking out 490 hp on a mild 454-cid Rat.

GM chose the Rochester four-barrel "Quadrajet" carburetor to fuel some of the best musclecars ever made and now 15 years later factory tuning is not available but still offered by Carb Shop. - Super Chevy Magazine » Read More